The present invention relates to a method and apparatus for drying onions.
After cutting, chopping, slicing, etc., onions tend to turn pink because of the bruising of the onion flesh that occurs during the cutting operation. This discoloration reduces the onion products' value and salability in the marketplace. In addition, "pinking" is accelerated by the conditions that exist during the early stages of dehydrating onion products. Therefore, pinking has been and continues to be a recurring problem encountered by onion dryers who dehydrate large volumes of onion pieces for sale.
When onions are dehydrated, the quality of the finished product is greatly influenced by the time-temperature conditions to which they are exposed. In general, to prevent pinking, low-temperature, low-humidity air, and relatively rapid initial drying are employed in an attempt to prevent the pink discoloration. Low temperatures result in extended drying times. Low-humidity air can only be maintained when recirculation is not practiced in the initial stages of the dryer and fresh air is circulated through the onions only once and then returned to the atmosphere thus making very inefficient use of the heat value of that air.
Also, relatively low loading of the dryers in the initial stages has been required to obtain rapid drying of the product; and yet the particle size of the onion pieces being dried could not be reduced to the point where the drying was particularly rapid. Normal slicing or dicing is conducted commercially, but the cutting instruments must be maintained extremely sharp to minimize bruising of the onions which increases their tendency to pink.
It can be easily seen that the above criteria previously required to prevent pinking tend to work at cross purposes to energy efficient onion dehydration. Therefore, if the pinking problem can be handled by a means other than controlling the drying parameters, a more efficient dehydration process for onions could be practiced.
Investigations into the mechanism of pinking have indicated that a complex series of reactions are involved which include the formation of at least one precursor. For the most part, these investigations have used onion purees and onion juice to study the pinking reactions. Various factors were identified in the studies which affect pinking in onions, including pH. However, none of the studies which considered pH to be a factor studied pinking during dehydration, and none even studied pinking of commercial onion products, only purees and juice.
Joslyn, et. al., "Reddening of White Onion Bulb Purees," AGRICULTURAL AND FOOD CHEMISTRY, 6, 754 (1958) at page 757, Column 2, states while discussing the effects of pH on pinking during storage that: "The complete loss in tendency to redden was accompanied by general deterioration in quality, softening of both outer and inner scale tissue."
Contrary to these teachings, applicants have found that treatment with a base to raise the pH of the onion pieces, at least on their surfaces, in conjunction with drying can be used to prevent pinking without the general deterioration in quality described in the art.
One advantage of the present process is that it allows a greater flexibility in the other drying parameters of the overall dehydration process.
A further advantage of the present process is that it allows the use of a greater proportion of recycled air during dehydration.
A still further advantage is that the present process minimizes the need for repeated sharpening of the onion cutting instruments to prevent bruising.
These and other advantages will become more apparent from the detailed description of the invention.